Incompressible Flow over Airfoils Questions and Answers

Answer: C

The new ratio is \(1.6/0.020 = 80\). Lift has doubled but drag has risen by a factor of two and a half, so the ratio falls despite the extra lift, which is why maximum lift-to-drag occurs at a particular moderate incidence.

Answer: D

Correct answer: The wing has a smaller lift curve slope than the section

The downwash induced by the trailing vortex system reduces the effective angle of attack at every section of a finite wing, so more geometric incidence is needed for a given lift coefficient and the slope is lower. The reduction grows as aspect ratio falls, which is why a slender high aspect ratio wing behaves much more like its section than a stubby one does.

Answer: A

Correct answer: The first digit gives the thickness as a percentage of chord

This is the false statement. In the four-digit series the first digit is the maximum camber as a percentage of chord and the second its chordwise position in tenths; thickness is given by the last two digits. So in NACA 2412 the camber is two per cent at four tenths chord, and the thickness is twelve per cent.

Answer: B

Correct answer: A generous leading edge radius and moderate camber

Low speed flight means operating at high lift coefficients and reaching high angles of attack, so what matters is a high maximum lift coefficient and a gentle stall. A generous leading edge radius keeps the suction peak moderate and gives a trailing edge stall with warning, and camber raises the maximum lift. Compressibility is not a concern, so thickness costs little.

Answer: C

Correct answer: A sharp leading edge and very low thickness

At supersonic speed the dominant penalty is wave drag, which grows with the square of the thickness ratio and is aggravated by any blunt shape that forces a detached bow shock. A sharp leading edge lets the shock attach, and minimum thickness minimises the wave drag. The low speed handling that results is poor, which is why such aircraft need high approach speeds or elaborate high-lift systems.

Answer: D

Correct answer: The leading edge radius

It is the leading edge radius that decides where separation begins. A generous radius keeps the nose suction peak moderate, so the boundary layer first fails near the trailing edge and separation creeps forward, giving a rounded lift curve and buffet warning. A sharp nose produces a violent peak and separation at the leading edge, and the lift collapses without warning.

Answer: A

Correct answer: Tailless aircraft, to reduce the nose-down moment

A cambered section has a nose-down pitching moment which, on a conventional aircraft, is balanced by the tailplane. A tailless design has no tail to do it, so the trailing edge is reflexed upward, loading the rear of the section in the opposite sense and bringing the moment coefficient near zero. It costs some maximum lift, which is the price of dispensing with the tail.

Answer: B

Correct answer: It is proportional to the angle measured from the zero-lift angle

Thin airfoil theory gives \( c_l = 2\pi(\alpha - \alpha_{L=0}) \) with the angle in radians. Camber enters only through the zero-lift angle, so it shifts the lift curve sideways without changing its slope - which is the theory’s most useful single result and the reason all thin sections share a slope of about 0.11 per degree.

Answer: C

Below the zero-lift angle the pressure difference reverses and the section pushes downwards, which is exactly how a conventional tailplane produces its download. Nothing about the section prevents it working in that sense.

Answer: D

Correct answer: Near the stall, where separation becomes significant

Over the linear range the theory works well because the flow is attached and behaves nearly as the inviscid model assumes. As incidence rises the boundary layer begins to separate, the effective camber is spoiled, and the measured curve bends away from the straight line before reaching a maximum and falling. Separation is a viscous phenomenon, and the theory has no viscosity in it.